Lower passenger density in first. A full plane flight produces x tons of CO2, y tons of NOx etc. There could be 600 economy seats or 100 first, or some other combination. If we all fly economy then there is less pollution per passenger.
Allegedly Emma Thompson offsets her CO2 impact but that doesn't help with NOx etc.
I'm also not sure there is enough planet available for everyone to offset their CO2 so it isn't really a solution.